A law enforcement source says the suspect in custody initially refused to cooperate.
The suspect had no identification when he was arrested and FBI found no information immediately in the system.
At least five people were killed in the Capital Gazette shooting, Acting Police Chief of Anne Arundel County Bill Krampf said in a press conference.
"There are five fatalities that is we know of," he said, adding that there were several other people who were "gravely injured."
Krampf promised more information in the next briefing. Another press conference is expected at 5 p.m.
These papers work hard for their communities. Capital Gazette Communications publishes multiple papers, including the Capital and the Maryland Gazette.
The publications have a rich history -- and they've continued to cover Annapolis, the Maryland state house, and state politics despite rounds of cutbacks over the years.
The staff does a lot with a little, in terms of resources. There are a couple dozen editors and reporters, some of whom work at the office every day. Others are out in the community at events and meetings.
The address where the shooting occurred was well known to readers and members of the public. In this case, shots rang out in the early afternoon, obviously a busy time of a day in and around a newsroom.
Steve Schuh, Anne Arundel county executive, told reporters that several people were killed and others were injured as a result of a shooting today at the Capital Gazette building in Annapolis, Maryland.
"We have had a terrible shooting incident here this afternoon," he said.
He added: "Those fatalities are so sad and I don't know what to say except our thoughts and prayers are with them and their families and we take comfort knowing they're in God's embrace."
Phil Davis, a crime and courts reporter at the Capital Gazette newspaper, just tweeted that he is safe and out the newspaper building.
Earlier today, Davis tweeted details about a shooting and said "there is nothing more terrifying than hearing multiple people get shot while you're under your desk and then hear the gunman reload."
"Can't say much more and don't want to declare anyone dead, but it's bad," he said.
A law enforcement source says a shotgun was used in the Annapolis shooting.
Multiple people were shot inside the newsroom at the Capital Gazette newspaper office in Annapolis, Maryland. Two law enforcement sources tell CNN there are multiple wounded and multiple fatalities.
Lt. Ryan Frashure, spokesperson of Anne Arundel County Police Department, told reporters that law enforcement officers are still in evacuating the building, following a shooting inside a newspaper building.
He confirmed that there were injuries, but did not expect it to be a mass casualty situation. Frashure said additional information about victims and the suspect will be provided once the building is completely secure.
Maryland Sens. Ben Cardin and Chris Van Hollen tweeted about the shooting at the Capital Gazette building in Annapolis.
Cardin, a Democrat, urged people to stay away from the area, and thanked first responders.
Van Hollen, also a Democrat, urged for people to "unite to end the violence."
One patient was taken to the University of Maryland Medical Center in Baltimore, Maryland, following the Annapolis shooting, according to Lisa Clough, the director of media relations for the hospital.
No information about patient, including his or her condition, is available at this time.
Clough says she does not know if they are expecting any additional patients.